Should Deputes Be Liable to Cover Classes?

Finding it difficult to find any official details as to whether Deputes (in secondary sector) should be available for please takes.
The authority I work in has cut back management time and PTs have had their notional management time cut by 1 period and are therefore more likely to be called upon to cover classes.
It appears that Deputes have escaped this and are, at present, exempt from covering for absent colleagues.
In order to raise the matter it would be beneficial to know the guidelines regarding such matters.
Any concrete advice (ie documents governing such things) would be welcomed.